Where is signed original Donitz's authorization to German representatives to ratify "Instrument of surrender signed at Berlin"?

As this question is now about the mere existence of a certain document from a row of events, and their validity, this answer kept growing due to the necessity to also incorporate circumstantial evidence. Dönitz signed two documents. One for Jodl, a quite different one for Keitel.

The official archive, the Bundes-Militärarchiv, Signature BArch RW44-I/37 would be the first port of call.
Online to be found here, with scans:

Die deutsche Kapitulation 1945
In der Galerie werden die im Bundesarchiv, Abt. Militärarchiv überlieferten Dokumente zur deutschen Gesamt-Kapitulation 1945 und den vorausgegangenen Teilkapitulationen gezeigt.

__The German Surrender 1945
In the gallery the documents handed down in the Federal Archives, Dept. Military Archives on the German total surrender in 1945 and the preceding partial surrenders are shown.

It might depend a bit on desired use for these. Wanting the "originals". But I guess they are not really on sale.

However, they are on record with:

"This is our Mona Lisa, so to speak," says Military Archives Director Michael Steidel, opening the cassette's closure with the signature RW 44-I. The original surrender documents that sealed the end of the Second World War come to light. They returned to Germany from the USA in the 1960s and have been kept in the Bundesarchiv-Militärarchiv since 1968, because of their special importance in a safe at 20 degrees and a humidity of 50 percent.

Only recently, says Michael Steidel, two museums asked again whether they could present the valuable documents in exhibitions. The military archives said no: "We won't give out the original, but we'd like to offer a reproduction. These documents have a material value that cannot be quantified," explains Steidel. A transport is too risky – for one of the most important constitutional documents in Germany.

In Freiburg, 55 kilometres of files from 1867 to the present are stored in five halls, including, for example, the plans of "Operation Walküre" of 20 July 1944. 4.5 kilometres are added each year, of which "only" one kilometre is archived after evaluation. […] The archive receives around 10,000 inquiries per year. It is accessible to everyone. Registration is required.
Frank Zimmermann: "Ein Archiv verwahrt das Kriegsende", NWZ, 2010.

_{Currently, some idiotic web-'designer' sold this office a 'solution' that effectively prevents access to the digital findbook.}

There is a little problem with this part from the question:

By the protocol of meeting Keitel was asked "do you have an authority to surrender)?" (means ReichsPresident agreement) He replied "yes". I suppose that he had an officially written Donitz agreement but possibly in one copy. When there were 4 files (for Russians, Americans, GB and France?) 3 of them got copies.

The authorisation had to be before the ceremony of signing the document. At that time the lines of communication were no longer free for German documents to travel fast.

When it became obvious that the Germans were stalling, Eisenhower threatened to close the western front to all surrendering Germans from the east.[17] Had this happened, German soldiers attempting to cross the line to surrender would be fired on and all subsequent surrenders would have to be to the Soviets. When Dönitz learned this, he radioed Jodl full powers to sign the unconditional German Instrument of Surrender at 1.30 am on the morning of 7 May. […]
Antonov stated that, while the internal discussions of the German military in no way obligated the Allied Powers, Jodl's signature could not be accepted as valid if he was doing so as Dönitz's representative since Dönitz himself was clearly acting in bad faith. He proposed that the definitive act of surrender should make it clear that the Commanders in Chief of each of the German armed services were, in signing it, surrendering their forces on the authority of the German High Command - and not as delegated by Dönitz or the purported Flensburg government.[…]
Complying with that demand Dönitz issued a telegraphed communication from his "Supreme Commander Headquarters" (Der Oberste Befehlshaber Hauptquartier) granting the necessary full powers, and accordingly the second Act of Military Surrender was signed by Field Marshal Wilhelm Keitel as Chief of the High Command of the Armed Forces and as acting Commander in Chief of the Army; by Admiral von Friedeburg as the Commander in Chief of the Kriegsmarine (navy), by General Hans-Jürgen Stumpff as Deputy Commander in Chief of the Luftwaffe (air force) as Field Marshal Robert Ritter von Greim, the Air Force Commander, had been injured. At the time specified, World War II in Europe ended. On 9 May, Dönitz issued orders to the German Armed Forces regarding the military surrender.
WP: Flensburg Government

The important distinction here is the Jodl had one kind of authority given that needed oral confirmation:

Since there was no way out of the dilemma, it was decided to send Jodl to explain with all force to Eisenhower ‘why a complete capitulation is impossible, but a capitulation only in the west would be immediately accepted’. In the early hours of next morning, 7 May, Jodl’s wire from Eisenhower’s headquarters brought the depressing news that the Allied Commander-in-Chief insisted that total capitulation be signed that day, otherwise all negotiations would be broken off. Eisenhower’s demand was seen in Dönitz’s headquarters as ‘absolute blackmail’ since if refused it would mean the abandonment of all Germans beyond American lines to the Russians. But with a capitulation to go into effect at midnight on 8/9 May, it would give forty-eight hours to extract at least most of the troops still fighting in the east. With a heavy heart, Dönitz therefore gave Jodl powers to sign the capitulation. At 2.41 a.m. on 7 May Jodl, in the presence of Admiral-General von Friedeburg, signed the Act of Military Surrender together with General Walter Bedell Smith and the Soviet General Ivan Susloparov in Eisenhower’s headquarters in Rheims. All military operations were to cease at 23.01 hours Central European Time on 8 May – given the hour’s time difference, a minute past midnight on 9 May in London. _{Die Niederlage 1945, pp. 432–3 (7.5.45). Eisenhower had given Jodl half an hour to reach a decision, but communications difficulties with Flensburg delayed the arrival of his message and receipt of Dönitz’s approval. – DZW, 6, p. 774. See also Bodo Scheurig, Alfred Jodl: Gehorsam und Verhängnis, Berlin and Frankfurt am Main, 1991, pp. 331–3.}
Quoted from Ian Kershaw: "The End. The Defiance and Destruction of Hitler’s Germany, 1944 – 45", Penguin, New York, 2011.

Take note that Dönitz himself later confirms that he gave out such an authority like he gave to Jodl orally, but to Keitel this was then in writing:

(Source freiheitistselbstbestimmtesleben.de, shady site)

The text is identical to National Archives Publication No. 46-4, United States Government Printing Office, Washington, 1945 or this National Archive version:

But after the last words of Keitel and this, a conundrum seems to resolve:

Dönitz received this information that morning, and, unwilling to concede to all the demands, decided to send General Jodl back to Eisenhower’s headquarters at Reims with yet another counter proposal. […] Jodl did not get very far with Eisenhower, and was simply informed that a surrender was to be signed on that very day under the terms laid forth the day before. […] This created a certain amount of activity in Flensburg, as Jodl had been chosen for the mission as one of the most outspoken opponents of unconditional surrender. Dönitz regarded it as unwise, for reasons cited in his May 1st radio addresses, and felt that forty eight hours was not enough time to withdraw forces from the Eastern Front still engaging the enemy. Dönitz also saw it as blackmail on the part of Eisenhower, and a violation of the Geneva conventions in that the Americans pledged to refuse the surrender of German soldiers should Germany fail to sign. After two hours of debate, at 12:40 a.m., Dönitz sent word to Jodl by a special radio link authorizing him to sign.
_{Jonathan Edward Klein: "At Zero Hour: The Government Of Karl Dönitz, With Reflections As Seen In German Literature", Thesis, Bowling Green State University, 2006.}

The Jodl paper above is not an authorisation for the unconditional surrender; it is "for an armistice" (with conditions that Jodl should negotiate with Eisenhower). Jodl then had to wire back to sign unconditional surrender nonetheless. According to Keitel, when he was in Karlshorst he then had indeed a signed paper from Dönitz with him as he was flown by UK plane to Berlin. That was then phrased as seen in the Dönitz letter above or as in the USNA version.

On 8th May, after Jodl's return from Eisenhower's headquarters at Reims, I was commanded by the Grand-Admiral, acting as Head of State and Supreme Commander of the Armed Forces—to fly by British transport aircraft to Berlin, with the preliminary instrument as signed by Jodl and Eisenhower's Chief of Staff.[…]
About an hour later, the general was back with the news that General Zhukov had agreed to twelve hours' grace being given, instead of twenty-four. He ended by asking for my credentials as the representatives of the victorious powers wished to inspect them; I would receive them back shortly. The signing was to take place 'towards evening', he added.[…]
They brought back my credentials and told me everything was in order, but apparently they still did not know at what time the Surrender was going to be signed.[…]
The ceremony began with a few introductory words; then Zhukov askedmewhetherIhadreadtheInstrumentofSurrender. Ireplied, 'Yes.' His second question was whether I was ready to recognise it with my signature. Again, I answered with a loud 'Yes!' The signing ceremony began at once, and, after I had been the first to sign it, the attestation. Finally, I and my party left the hall by the door close behind me.
Wilhelm Keitel: "In the Service of the Reich. (originally published as The Memoirs of Field-Marshal Keitel", edited with an Introduction and Epilogue by Walter Görlitz Translated by David Irving, Stein & Day: New York, 1979 (1966), p230–231.

So Keitel had one document from Dönitz with the signature of the OKW chief authorising the ratification of an unconditional surrender. Doubting its very existence at the time within the framework of a conspiracy theory with certain motives seems quite incompatible with the evidence available. Dönitz confirmed it, Keitel confirmed it etc.